Analysis of the ability of titanium dioxide nanoparticles to produce singlet oxygen and hydrogen peroxide under ionising radiation

Abstract

Radiotherapy is a vital component in the treatment of malignancies, with approximately 50 to 70% of cancer patients receiving it at some point. This therapy consists of irradiating the affected body part with an external beam of ionising radiation. A limitation of this technique is its damaging effect on healthy cells surrounding the tumour. Semiconducting nanoparticles could offer a solution to this, as these compounds can generate reactive oxygen species (ROS). ROS are a group of free radicals that are known to be able to damage DNA and potentially cause necrotic or apoptotic cell death. Using these metal nanoparticles could increase the radiation damage to malignant cells during radiotherapy, without raising the risk of cell death for the surrounding tissue. Especially TiO2 nanoparticles have been increasingly studied due to their ability to produce singlet oxygen, one of the most reactive ROS, while also having low toxicity and cost. However, many aspects of the ROS-formation remain unclear, including certain ROS production pathways. This report will therefore focus on the ability of TiO2 to produce specific ROS in response to ionising radiation, which will primarily be compared to the ROS formation through the radiolysis of water. The focus lies on the detection of singlet oxygen and hydrogen peroxide (H2O2 ) after irradiation by either a Co-60 gamma or a low energy X-ray source over a range of 5 to 40 Gy. Their concentrations are detected by fluorescence spectroscopy for singlet oxygen, and by UV-VIS spectroscopy for hydrogen peroxide. The results were promising for the use of TiO2 in radiotherapy, with both MilliQ water and TiO2 being able to produce singlet oxygen in increasing amounts over the entire researched irradiation range for both radiation sources; Moreover, TiO2 could produce a significantly larger amount of this ROS than the MilliQ water samples did. The hydrogen peroxide detection experiments were first conducted with ZnO instead of TiO2 . During these, the trends observed for the H2O2 concentration were similar to the ones obtained during the singlet oxygen experiments. The main difference was that the ZnO samples produced approximately the same amount of H2O2 as the MilliQ water samples for X-ray irradiation, instead of higher values. Lastly, the hydrogen peroxide concentration in both TiO2 and ZnO samples was measured after irradiation by Co-60. ZnO exhibited an upward trend in which the H2O2 molarity increased as the applied dose got higher. The detected H2O2 levels in the TiO2 samples were, on the other hand, relatively stagnant. This observation is likely attributed to a relatively high extent of either the degradation, conversion, or adsorption of this ROS at the surface of TiO2